Devices employing optical signals are widely used, and these devices often use multiple stage arrays of optical switches to direct signals where they are needed. Frequently, a signal must be routed to one of a number of alternative outputs, and this routing is conveniently accomplished by supplying the signal to an input of a switch array and then setting switches of the array to route the signal to the desired output. Switch arrays may conveniently be constructed of binary switches, each binary switch having a single input and two outputs. Through the use of appropriate combinations of such switches, it is possible to construct an array that allows a signal to be routed to one of any number of alternative outputs, and to transfer the signal from one output to another as needed.
Thermo-optical binary switches are available that have three states. Each state requires a different power level. A first state is a “cold” state in which the application of power is very low or nonexistent. In this state, the switch routes the signal to a first, or “up” output. This state may conveniently be referred to as an “up” state. In this state, the switch directs a signal to a first output. The signal directed to the first output is at substantially the same level as the signal at the input, but a small signal resulting from crosstalk or leakage may also be present at the second output. A suitable standard for such a crosstalk signal for a switch in the “cold” state is that it be at a level representing an attenuation of over −10 dB. That is, the crosstalk signal must be less than 10% of the input signal.
A second state is a low power, or “off” state in which the application of power is low, but not so low as in the “cold” state. This state may also be referred to as an “up” state, because a switch in this state routes a signal to the “up” output. The signal directed to the first output is at substantially the same level as the signal at the input, but a small signal resulting from crosstalk is present at the second output. This crosstalk signal is at a very low level, and may be at a level representing an attenuation of over −20 dB. That is, the crosstalk signal must be less than 1% of the input signal.
The third state is a high power, or “on” state. In this state, the switch directs a signal to the second output, which may also be referred to as a “down” output. The signal directed to the second output is at the same level as the signal at the input, but a small signal resulting from crosstalk is present at the first, or “up” output. A suitable requirement for this crosstalk signal is that it meet the same restrictions as the crosstalk signal produced at the “down” output of the switch when the switch is in the “off” state. That is, the crosstalk signal must be at a level representing an attenuation of over −20 dB, or less than 1% of the input signal.
The array output to which the input signal is routed at any particular time under consideration may be referred to as the bright output. All other array outputs may be referred to as dark outputs. It is important to insure that signal levels at the dark outputs are at a very low level, in order to prevent errors resulting from misinterpretation of crosstalk or leakage signals. Frequently, multiple switch arrays are used in an application with one output of each of a number of switches being connected to a multiplexer. If crosstalk or leakage signals at the switch array outputs are not properly constrained at a desired low level, multiple crosstalk signals at a multiplexer or similar device may be misinterpreted, causing the device to produce a spurious output signal.
In order to achieve a desired attenuation of signal levels at the dark outputs, shutters may be used. Typically, a shutter is used for each array output, with each shutter having an input connected to a switch output, with the output of a shutter forming an associated array output. Each shutter may be in a “cold”, “off”, or “on” state. The “off” states may also be referred to as “up” states, and when the shutter is in an “up” state the signal is blocked. “Cold” or “off” shutters introduce attenuation similar to that introduced by “cold” or “off” switches, respectively. When a shutter is in an “on” or “down” state, the shutter passes the signal substantially without attenuation.
It is highly desirable to direct the signal to a desired output of an array, while minimizing the signal levels appearing at other outputs of the array. During stable operation of a switch, that is, after a sufficient time has passed following a switch transition, the above description of the switch states is accurate. However, during a switch transition, the signal level at each output cannot be reliably predicted. During a transition, the output at each switch may range from the same level as the input signal, down to the crosstalk signal level prescribed for stable switch states. In switch arrays, the outputs of switches are frequently used as the inputs of other switches, and unpredictability of signal levels during switch transitions may be amplified as unknown signals are applied to switches which then route the signals in unknown ways. After switching is finished, the outputs of the switch array will be stable and predictable, but significant uncertainties may prevail during transitions. In addition, optical switch arrays are often used in very small devices, where minimizing heat dissipation is highly desirable, leading to a need for power conservation in the design and operation of the switch arrays.
There exists, therefore, a need for systems and techniques for construction and management of arrays of binary thermo-optical switches that consistently reduce transient effects and power dissipation experienced by the arrays, and for arrays of switches that operate so as to reduce transient effects and power dissipation.